The joinder of two shaft segments has been the subject of extensive development. The objective is the same in all cases, which is to join two shaft segments so that one can drive the other, and so they can not be pulled apart. The general concept is somehow to hold the segments together by means such as engaging threads, or headed shapes which are somehow engaged. Relative rotational movement of the segments is often attended to by splined engagements.
Despite widespread development, problems continue to arise to be solved. Some of these are the problems of wear, fatigue, and stress concentrations in the shafts, all of which should be minimized or avoided entirely. It is an object of this invention to provide a simply constructed coupler devoid of stress concentration points in the assembly, which tends to be self-centering to a stable position, all in a simple construction.